2000ApJ...535..275H

Infrared Space Observatory (ISO) 2-45 µm observations of seven proto-planetary nebulae (PPNs) and two other carbon-rich objects are presented. The unidentified emission features at 21 and 30 µm are detected in six sources, including four new detections of the 30 µm feature. This previously unresolved 30 µm feature is now resolved and found to consist of a broad feature peaking at 27.2 µm (the ``30 µm'' feature) and a narrower feature at 25.5 µm (the ``26 µm'' feature). This new 26 µm feature is detected in eight sources and is particularly strong in IRAS Z02229+6208 and 16594-4656. The unidentified infrared (UIR) emission features at 3.3, 6.2, 7.7, and 11.3 µm, which are commonly observed in planetary nebulae and H II regions, are also seen in these PPNs. However, their strengths relative to the continuum plateaus at 8 and 12 µm are weaker than in planetary nebulae. The 6.9 µm feature, seen almost exclusively in PPNs, is strong. New millimeter CO and HCN observations were made; they support the carbon-rich nature of the objects and yield the expansion velocities of the gaseous envelopes. The spectral energy distributions of these PPNs were fitted with a radiative-transfer model, taking into account the emission features at 21, 26, and 30 µm. A significant fraction of the total energy output is emitted in these features: as high as 20% in the 30 µm feature and 8% in the 21 µm feature. The fact that so much energy is carried in these features suggests that the material responsible for these features must be made of abundant elements and most likely involves carbon. SiS₂ appears to be ruled out as the emitter of the 21 µm feature due to the absence of a predicted companion feature.